1. Field of the Invention
The present invention relates generally to global positioning system (GPS) equipment and methods, and more particularly to separating GPS receiver front-ends from GPS processing such that the GPS front-ends may be placed in expendable launch vehicles, such as radiosondes and sonobuoys.
2. Description of the Prior Art
Weather balloons typically carry aloft radio transmitters that broadcast humidity, temperature and atmospheric pressure at various altitudes of a flight. Such instruments are called radiosondes, and when tracked by transducers to gauge wind velocities, the radiosondes are referred to as rawinsondes. Balloon launched rawinsondes (sondes) are used throughout the world on test ranges to measure and radio-telemeter the balloon's position, altitude, upper atmosphere wind velocities, air temperature, relative humidity and barometric pressure. Such data supports meteorological reporting for the National Weather Service and is used on Department of Defense (DOD) test ranges to validate vehicle wind load limits on days scheduled for launches.
The prior art measures balloon position using several techniques, e.g., a transponder and radiotheodolite, long range navigation (LORAN) and OMEGA, none of which are capable of precise altitude and latitude/longitude resolutions. The radiotheodolite technique requires manning of ground equipment. OMEGA has poor geo-location capability. The reliable reception range of LORAN-C navigation data can be reduced by hundreds of miles during thunderstorm activity. Rain showers, wet fog and snow flurries along a LORAN-C chain can produce "precipitation static" which degrades LORAN-C reception. External man-made interference is also a problem at the 100 KHz frequency used by LORAN-C, due to the long range propagation characteristics of such low frequency signals.
Among existing long range navigational aids, OMEGA, LORAN, TRANSIT and global positioning system (GPS), the GPS system obtains the best accuracies. GPS signals are inherently immune to interference, in part due to its direct sequence, spread spectrum, signal structure and a line-of-sight radio signal propagation characteristic of the two GPS satellite's carrier frequency signals, L1 and L2. Code division multiple access (CDMA) is used to separate signals from the individual GPS satellites. Each GPS satellite transmits a pseudo-random number (PRN) key that is needed by the receiver to decipher information. The L1 GPS band at 1,575.42 MHz, also provides a degree of immunity to terrestrial signals at long ranges, which are over-the-horizon. The GPS L1 C/A signal occupies slightly less than two MHz of bandwidth. The accuracies needed in rawinsonde applications can be obtained worldwide with GPS. Positional accuracies on the order of fifteen meters are possible when using a differentially corrected C/A signal during periods the DOD has engaged a deliberate dither called Selective Availability (SA). Under normal operating conditions, the DOD introduces errors via SA into the GPS system so that unauthorized receivers cannot use the GPS system at its most precise levels of accuracy against the United States or its armed forces in military actions.
In conventional GPS receivers, navigation signals are continuously processed in real-time. A conventional GPS receiver tracks a plurality of PRN phases corresponding to multiple GPS satellites, all in real-time. The ephemeris and catalog data are also extracted in real-time. GPS ephemeris data informs a GPS digital signal processor of the precise orbit of a corresponding GPS satellite. Such real-time, wide-bandwidth tracking of multiple satellites necessitates sophisticated and complex hardware and software.
A major drawback to GPS systems, however, are their cost. Such systems are too expensive to be considered disposable or expendable. It would be prohibitively expensive to most weather data gathering institutions to fly a complete GPS receiver aboard a non-recoverable sonde. The same is true for sonobuoy launchings. GPS receivers are expensive because of the complex signal processing required to extract the positional data from the GPS signal.
One approach to making GPS systems affordable in launch vehicles, e.g., rawinsondes and sonobuoys, is to concentrate the overall system costs in the centralized ground processing equipment. A rawinsonde or sonobuoy could be adapted to relay the raw GPS signals it receives over a radio band, e.g., 1600 MHz or 400-406 MHz which are reserved for meteorological use. Relaying uncorrelated GPS signals to a ground station for GPS processing there would involve a minimum of equipment in the launch vehicle. But a two MHz downlink channel, for example, in the meteorological band, would be needed and would be susceptible to interference. This technique would also require a relatively high power relay transmitter due to the inherent wide bandwidths of the GPS signals and their CDMA spread spectrum modulation.